A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the wafer may be limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers with a wavelength larger or equal to 193 nm, it has been proposed to use shorter wavelength radiation of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include laser-produced plasma sources, discharge plasma sources or synchrotron radiation from electron storage rings. Other proposed radiation types include electron beams and ion beams.
When using electron or ion beams, or EUV, the beam path, including the mask, substrate and optical components, should be kept in a vacuum to prevent absorption and/or scattering of the beam. A total pressure of less than about 10−6 mbar may be used for electron and ion beams. Optical elements for EUV radiation can be spoiled by the deposition of carbon layers on their surface, so hydrocarbon partial pressures should generally be kept as low as possible, and layers of carbon from the mirrors may need to be cleaned from the mirrors from time to time. For a lithographic apparatus using EUV radiation, the total vacuum pressure may be higher than the pressure used for electron and ion beams, which would typically be considered a rough vacuum.
To be able to image the mask onto the substrate, a projection system is used which in the case of EUV may comprise one or more mirrors held by a frame. The features that may be imaged by the projection system may be smaller than 100 nm and therefore the image is very sensitive to aberrations of the mirror and deformations of the frame. The deformations and aberrations may be caused by thermal fluctuations of the low expansion material that is used for the mirrors and the frame. The fluctuations may be caused by heating and or cooling of the projection system. Heating may occur during illumination of the mirror with EUV light and during cleaning of the mirrors with a relatively hot gas. Cooling may be caused by adiabatic expansion during pump down of the vacuum chamber.
Since the projection system is kept at a vacuum, cooling or heating of the mirror mainly occurs by radiation of heat to or from the vacuum wall of the apparatus and to other components within the vacuum environment. The transfer of energy out of or into the vacuum chamber by radiation takes a lot of time in which the lithographic projection apparatus may not be used.